tests: use system time in milliseconds to generate keyspace name suffix

There are test suites where test cases share the same keyspace, but few of
cases require a dedicated keyspace with specific parameters.

The change allows producing unique keyspace name based on system time in microseconds,
to ensure the unique name for paralelized tests.

scylla/tests/common/utils.rs

@@ -42,7 +42,7 @@ pub(crate) fn unique_keyspace_name() -> String {
         SystemTime::now()
             .duration_since(UNIX_EPOCH)
             .unwrap()
-            .as_secs(),
+            .as_micros(),
         cnt
     );
     println!("Unique name: {}", name);